1. Field of the Invention
The present invention relates to new crystalline hot melt adhesives containing isocyanate groups.
2. Brief Description of the Prior Art
Methods of producing joints by means of solvent-free hot melt adhesives having a solids content of 100% are nowadays becoming increasingly important since the use of systems containing solvents require expensive plants for the recovery of the solvents and the use of aqueous dispersion or solution systems requires evaporation of the water, which may also be highly cost intensive in some circumstances. Hot melt adhesives offer a technological alternative which is already widely used. Their advantages lie in the fact that when applied as hot melts, they rapidly solidify on cooling and build up strength. One disadvantage is that the bonding of heat-sensitive substrates is made difficult by the high melting temperatures of these adhesives. Also, if the adhesive is not further processed as soon as it has been applied, it rapidly solidifies into a state in which it can no longer be wetted and can then only be reactivated by heat under extreme temperature conditions. Further, due to the thermoplastic character of the hot melt adhesives, their bonds have only a limited heat distortion temperature range.
An elegant method by which the properties of hot melt adhesives can be obtained by building up strength on cooling can be utilized at low temperatures but the bonds produced by the adhesives will nevertheless have high heat strength. Such a bonding process is already described in principle in DE-PS 878,827 and DE-OS 2,609,266, which uses reactive hot melt systems based on isocyanate-containing prepolymers of diisocyanates and polyester diols having melting ranges above 40.degree. C. Owing to their low molecular weights, the products are liquid and processible at temperatures only slightly above the melting range of the polyesters and attain a sufficiently high molecular weight after a chain lengthening reaction has taken place on the substrate. Combined with the recrystallization of the crystalline soft segments, this results in an increased initial strength while the final strength is achieved by complete reaction of, for example, atmospheric moisture with the free isocyanate groups still present, to form linear, high molecular weight polyurethane polyureas. In the very rapid cycles nowadays customary in industry, it is precisely the necessary pre-lengthening of the isocyanate-containing prepolymers which constitutes a step which is difficult to reproduce and which, if not sufficiently completed, results in faulty adhesive bonds owing to the low cohesive strength, whereas if carried too far has the result that the adhesive layer obtained can no longer be sufficiently activated because the molecular weight is already too high.
Another means of producing reactive hot melts is described in EP-A 340,906. The hot melts mentioned there appear to fulfill the requirements for rapid cycles, but the products are expensive to produce due to the use of two amorphous prepolymers differing in their glass transition temperatures, and further, the viscosities of the products are higher owing to the higher glass points, so that higher processing temperatures are required for sufficiently wetting the substrates.
DE-P 3,932,018.9 is aimed in a similar direction, namely the rapid attainment of a higher initial strength. According to the said specification, more rapid crystallization is achieved by mixing two polyesters having differing melting points. Reactive hot melts based on relatively high melting polyesters are described in EP-A 354,527. Systems of this type based on, for example, dodecanoic diacid hexanediol polyesters crystalline very rapidly but are liable to form very brittle systems in the fresh state. This disadvantage is overcome in the products described in DE-P 4,005,390.8 and P 4,016,717.8 but even these products are limited in their capacity for crystallization at elevated temperatures.
Apart from this physical behavior of the adhesive applied to its substrate, the dosed application of the hot melt adhesives is a problem, particularly when the surfaces of the substrates are not planar so that the adhesives cannot be applied by means of, for example, rollers or broad sheeting dies. Various systems have been offered by the industry to deal with this problem. The application of a spun bonded material is particularly suitable for controlled application of the adhesive to a substrate which may be irregular. For such forms of application, the adhesive is required to have certain rheological properties to ensure a problem-free application in practice. It would therefore be desirable to have a bonding process which is technically simple to realize and can be incorporated in rapid machine cycles even under unfavorable conditions and yet can be carried out with relatively low melting adhesives capable of being activated at moderate temperatures.